(1) Field of the Invention
This invention relates to improvements in a process for the dehydration of water-containing coal by heating a slurry of the water-containing coal and a solvent, or improvement in a process for the preparation of a slurry of dehydrated coal and a solvent. It also relates to the use of a slurry of dehydrated coal so prepared as a starting material for the liquefaction of coal and to an improved process for the liquefaction of water-containing coal.
(2) Description of the Prior Art
After being mined, coal is subjected to a coal preparation process for removing ash as completely as possible. This coal deashing process is usually carried out by use of water, and the separation efficiency becomes higher as the particle size of the coal is reduced. However, the amount of adhesive moisture contained in the deashed coal increases in inverse proportion to the particle size of the coal. Usually, this adhesive moisture amounts to 20% or more. Moreover, some kinds of brown coal have a water content of as high as 20-65%.
Among the well-known methods for drying such water-containing coal are (a) flash drying and (b) drying in oil. Flash drying is being employed in East Germany, Australia and the like, but has the disadvantage that the coal is oxidized in the course of drying. Oxidized coal is subject to spontaneous ignition and its storage involves considerable difficulty. Moreover, when used as a starting material for the liquefaction of coal, such oxidized coal gives only a low degree of dissolution and causes an increase in the hydrogen consumption which has an important influence on the economical efficiency of the coal liquefaction process. On the other hand, drying in oil causes no oxidation of the coal. A number of prior art methods based on the principle of drying in oil are disclosed in Japanese Patent Laid-Open Nos. 112902/'78, 125406/'78 and 66904/'79. According to any of these methods, however, the dehydrated coal still has a water content of about 10%. When the coal so treated is transported as fuel, such a high water content uneconomically causes an increase in transport cost. Moreover, when the coal so treated is used as a starting material for the liquefaction of coal, the decrease in the net supply of coal causes a reduction in equipment capacity and the increase in the partial pressure of water vapor causes a decrease in the partial pressure of hydrogen. As a result, the coal liquefaction equipment need have higher pressure resistance and hence involves uneconomically increased costs.